Answer: If the fruit is beginning to fall, it's probably ripe. Obviously, the riper the fruit, the sweeter the taste. But, overripe fruit won't store well. I'd pick those that are fully colored, with a slight "give" when grasped. If they're still hard, wait a few days and test again. A general rule of thumb, however, is to harvest just after the first fruits fall from the tree. Experience will tell, so enjoy your plums, and next year you'll be able to tell by the color when the fruits are ripe.
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